Mechanical face seals are commonly used in fluid devices, such as pumps or hydraulic motors, where a rotating shaft extends through a housing, for preventing leakage of fluid along the shaft in to or out of the housing. Such seals are principally comprised of two elements, having complementary radially extending planar surfaces which mate to form an area of contact that blocks leakage of the fluid.
For example, to seal an annular gap about a rotating shaft extending through a pump housing, a typical mechanical face seal includes a non-rotatable seal element attached to the housing, and a rotating mating ring element fixedly attached to the shaft. The rotating and non-rotating elements of the seal are held in contact with one another by a spring acting on one of the seal elements, to thereby block leakage through the annular gap of a fluid contained within the housing.
For such a face seal to effectively block leakage, therefore, the mating elements must be held in virtually intimate contact. This creates a problem in that when the shaft rotates, friction between the mating elements tends to cause wear and generates heat in the seal elements.
The seal elements are, therefore, generally constructed of specially selected compatible material pairs that in combination provide low friction and long life. For example, it is common to utilize a relatively soft material with inherently high lubricity, such as carbon graphite, for one mating element, in combination with a relatively hard material, such as silicon carbide, for the other mating element of the seal. Both elements of the pair are also typically ground to be optically flat throughout the area of contact to ensure a fluid tight seal and to reduce operating friction to a minimum.
A thin film of fluid between the mating surfaces in the contact area of the seal is generally provided to further reduce operating friction. In a pump, for example, the radially outer extent of the actual contact area between the rotating and stationary elements of the seal is typically exposed to the fluid within the pump housing. In contrast, the radially inner extent of the contact area is exposed to lower, sometimes ambient atmospheric, pressure around the shaft. If the fluid inside the housing is pressurized, a small amount of fluid will force its way into the contact area between the seal elements to form a lubricating film. Furthermore, even absent any significant pressure differential from inside to outside of the housing, relative motion between the mating elements as the shaft rotates, will tend to pump a small amount of fluid into and out of the contact area, to thereby providing the desired thin lubricating film of the fluid being sealed.
It must be understood, however, that although friction between the mating elements can be reduced by taking measures such as those described above, some friction must inherently remain in the contact area to keep the seal elements in intimate contact with one another for maintaining the fluid seal. Unfortunately, this inherently necessary friction generates heat in the mating elements, and in the lubricant film separating them, as the fluid device is operated. Such heat generation is a serious concern in the design of any mechanical face seal. This is particularly true for face seals which must operate at high rotational speeds such as, for example, several thousand rpm, and have relatively small surface areas available for transferring heat to a cooling fluid.
Unless some mechanism is provided for effectively removing the heat generated during operation, the temperature of the mating elements will rapidly increase to several hundred degrees above the average temperature of the fluid being sealed. Such an increase in temperature has two primary detrimental effects. First, excessive temperatures tend to physically destroy the seal elements themselves. Large temperature gradients will quickly develop in the seal elements between the contact area and portions of the elements which are located some distance away from the contact area. Large temperature gradients in materials such as silicon carbide can lead to fracture of the material. Warpage of the seal elements may also occur, thereby destroying the ability to prevent leakage. Second, high operating temperatures in the mating elements may cause vaporization of the lubricant film in the contact area between the mating surfaces, which leads to significant increases in friction and heat generation in the contact area. Such vaporization is particularly problematic in face seals which must operate with volatile fluids such as propane, liquid natural gas, propylene, or other fluids that tend to vaporize or combust at relatively low temperatures.
In summary, therefore, high operating temperatures accelerate wear in the seal elements, which in turn leads to the necessity for more frequent overhaul and repair, thereby significantly increasing operating costs. To achieve long operating life some effective mechanism must be provided for removing heat inherently generated during operation of the face seal.
Prior mechanical face seals have taken numerous approaches to solving lubrication and cooling problems such as those described above. Some rely solely on directing coolant into the actual area of contact between the seal elements via fluid passages extending through the stationary seal element. British patent 811,299 to Horsley is illustrative of this approach. This approach has proved to be ineffective in some instances, with respect to heat dissipation, for two reasons. First, the interior of the rotating element is not cooled. This results in uneven cooling of the rotating seal element, which creates unacceptably high internal stresses within the material of the rotating element. Such internal stresses can cause premature failure of the rotating element. Second, because the mating surfaces need to remain in virtually intimate contact in order to maintain the fluid seal, the maximum rate at which coolant can flow through the contact area is often too low to provide effective cooling. Pumping more fluid into the contact area for cooling purposes would require that the effective gap between the mating surfaces be made wider to accommodate the additional flow. As the flow into the gap is increased to meet demands for adequate cooling, the gap widens proportionately, and eventually opens an unacceptably large leakage path past the seal.
Other approaches have utilized fluid passages in the rotating elements in addition to passages through the stationary elements, but have provided inadequate cooling because they, like the previously described approach, direct the flow of coolant directly into the gap between the contact surfaces. U.S. Pat. Nos. 3,675,935 to Ludwig and 4,961,678 to Janocko employ this approach.
In one approach commonly applied to seals having elements which rotate at low speeds and have large areas or light loads, the seal is mounted in a wash plenum which is filled with a fluid used for cooling and lubricating the seal. As the seal elements rotate, heat is dissipated into the surrounding fluid through essentially static conduction and convection. Rotation and pressure differentials across the seal also cause a small amount of fluid to be drawn into the contact area for lubricating the seal.
At high rotational speeds, however, this approach does not work well, if at all. The rotating elements of such seals begin to act as centrifugal pumps, pumping or slinging fluid away from the contact area. Without a supply of coolant adjacent the contact area, friction and seal temperatures rise rapidly. An area of low pressure adjacent the contact area can also develop, causing volatile fluids to flash to a vapor state. Once the fluid around the contact area is replaced by vapor, seal friction, heat generation, and temperature rapidly increase, resulting in increased wear and reduced life of the seal.
The problems and inadequacies associated with prior approaches to cooling mechanical face seals are exacerbated in some fluid devices by a standard recently promulgated by the American Petroleum Institute (API). API Standard 682, entitled Shaft Sealing Systems for Centrifugal and Rotary Pumps, requires that in pumps subject to the standard which utilize a silicon carbide mating ring, the faces of the mating ring shall not be clamped, in order to facilitate repair and to preclude fracture of the mating ring due to clamping stresses. Generally, compliance with API 682 requires that the mating ring be carried as a loose fitting piece in a mating ring carrier element which is affixed to the rotating shaft, and forms part of the rotating element of the seal. Because the fit between the mating ring insert and carrier is loose, the ability to transfer or spread heat by thermal conduction from the insert into the carrier is significantly reduced.
Accordingly, further improvements are necessary to maximize coolant and lubrication fluid flow while maintaining a fluid seal. It is an object of our invention, therefore, to provide a mechanical face seal with improved life by enhancing heat dissipation and lubrication. Further objects of our invention include providing:
1) a mechanical face seal as above with improved fluid sealing capabilities; PA1 2) a mechanical face seal as above having improved life, thereby decreasing overhaul frequency, and operating expense; PA1 3) a mechanical face seal as above with improved performance at operating speeds as high as 35,000 RPM; PA1 4) a mechanical face seal as above with improved performance in applications involving volatile liquids such as propane, liquid natural gas, and propylene; and PA1 5) a mechanical face seal having improved cooling of a loosely fitting silicon carbide mating ring meeting the requirements of fluid devices subject to API Standard 682.